FIELD OF THE INVENTION
This invention has as its object an integrated magneto-optical read and write head and a process for producing such a head.
Magneto-optical devices generally comprise three essential components, namely a recording medium, an optical head and a magnetic head;
The recording medium, of a magneto-optical material, is capable of setting, in the form of magnetic domains, binary data corresponding to the orientation of the magnetization of the material, either to the right or to the left (for so-called parallel recording), or up or down (for perpendicular recording). The domains are of small size (about 1 square micron).
These domains are able to be oriented in the same direction as an outside magnetic field if the latter is greater than a specific, critical field of the material, known as the coercive field and designated Hc.
This coercive field represents, as it were, the repatance to the orientation variation of the domains under the effect of an outside field.
For a domain oriented in a direction arbitrarily considered as positive, an outside field can switch this domain to a negative orientation only if this field takes a lower negative value -Hc.
To reorient the domain in the positive direction, the outside magnetic field will have to increase to take a positive value greater than +Hc.
A second property of a magneto-optical material is that its coercive field decreases when the temperature rises from the ambient temperature (20.degree. C.) to a value between, for example, 100.degree. C. and 300.degree. C.
This reduction of the coercive field with temperature makes it possible to control the switching of the magnetization. For this purpose, an outside magnetic field is applied having a direction that is opposite the initial orientation of the medium and having a value greater than the coercive field when hot but less than the coercive field at the ambient temperature. A laser beam is focused on the zone to be treated to heat the material locally.
A third essential property for a magneto-optical material is to cause a good optical detection signal. This detection signal is generally due to an angular rotation of the polarization plane of the light wave during its reflection on the material. The value and the sign of this rotation are different in the direction of the orientation of the magnetization which will be shown by a "1" or a "0".
This rotation can occur at the interface of the magometo-optical material with air (KERR effect) or while passing through the material (FARADAY effect).
In the case of a transparent material deposited on a reflector, the signal is a response to the two KERR and FARADAY effects combined.
The detected magneto-optical signal is proportional to the angle of rotation of the polarization plane in the reflection on the material.
The optical head performs two functions, one of writing, the other of reading:
In writing it must focus the light radiation coming from a light source (laser diode, for example) on a surface that the smallest possible area to heat a zone of the medium and only this zone to cause a switching of the magnetization under an outside magnetic field: PA1 a first optical subassembly comprising an optical input able to receive an optical radiation, an optical output placed in an output plane, first optical guide means between the input and the output, a photodetector, second optical guide means between the output and the photodetector, PA1 a second magnetic subassembly, comprising a magnetic circuit with at least one polar part placed in a plane perpendicular to the output plane, this polar part exhibiting a magnetic pole turned toward the recording medium, an electrically conductive coil surrounding said magnetic circuit, this head being characterized by the fact that the magnetic pole is set back relative to the optical output plane, the optical output of the first subassembly and the magnetic pole of the second subassembly exhibiting a crosswise offset.
In reading it must analyze the beam reflected by the medium and show the rotation of the polarization plane of an electric signal variation, with photodetectors, for example.
A magnetic head is necessary, in writing, to apply a switching field to the material. The value of the field will have to be sufficient but remain less than the coercive field of the material at the ambient temperature, so as not to erase or degrade the existing recordings on the adjacent tracks.
The direction of the magnetic field is the direction that it is desired to impart to the domains during the writing.